The present invention generally relates to methods and apparatuses for projection printing, and more particularly to a method and apparatus for projection printing suited for application in a projection printing (exposure) on a wafer or a mask when producing a semiconductor device.
A reduction projection printer (stepper) generally comprises a mercury-vapor lamp, a reducing lens, an X-Y stage, and a chuck. A wafer is secured on the chuck which is fixed on the X-Y stage, and an integrated circuit (IC) pattern on a reticle is reduced via the reduction lens and an image thereof is formed on the wafer to carry out the exposure. In order to form a circuit pattern with a high accuracy by a developing process which is carried out after the exposure process, it is necessary that the illuminance has a predetermined value and is uniform within a predetermined exposure region on the wafer.
Accordingly, when an inconsistent exposure is detected in the circuit pattern after the exposure and developing processes or the mercury-vapor lamp is changed, an illuminance measuring apparatus is used to measure the illuminance and illuminance distribution. A conventional illuminance measuring apparatus comprises a substrate and five photodetectors arranged at predetermined locations on the substrate. The illuminance measuring apparatus is inserted into the reduction projection printer in place of the reticle when measuring the illuminance, and the illuminance and illuminance distribution at the IC pattern of the reticle are measured from outputs of the photodetectors.
However, the light emitted from the mercury-vapor lamp passes through the reticle and the reduction lens before being focused on a predetermined exposure region of the wafer. Accordingly, it is impossible to know accurately from the measured results obtained at the reticle position the illuminance and illuminance distribution in the predetermined exposure region. Hence, there is a problem from the point of view of controlling the illuminance in the actual exposure region on the wafer. In addition, the characteristics of the photodetectors are not perfectly identical among the independent photodetectors, and for this reason, the reliability of the measured results obtained from the conventional illuminance measuring apparatus is unsatisfactory.
Accordingly, in addition to the measurement made in the illuminance measuring apparatus, an open frame test is conventionally carried out to detect the illuminance distribution on the actual wafer surface. The open frame test is carried out by exposing a dummy wafer which has a photoresist layer formed thereon, and changing the exposure time so as to detect the illuminance distribution on the water surface. However, according to such a method of measuring the illuminance, it is necessary to carry out the open frame test for detecting the illuminance distribution on the wafer surface in addition to measuring the illuminance at the reticle position. As a result, there are problems in that the processes of obtaining the illuminance and illuminance distribution in the predetermined exposure region are complex and troublesome to carry out.
Furthermore, when inserting the illuminance measuring apparatus (substrate having the photodetectors) into the stepper in place of the reticle, a platten (base) on which the reticle is placed may become scratched and damaged. There is also a problem in that dust particles and the like easily enter into the stepper at the insertion and extraction of the reticle and the illuminance measuring apparatus. Moreover, the operation sequence of removing the reticle from the stepper, inserting and then removing the illuminance measuring apparatus from the stepper, and again inserting the reticle into the stepper is extremely troublesome to carry out and is also time consuming. Normally, the serviceable life of the mercury-vapor lamp is 400 hours to 500 hours, and there is a problem in that the exposure efficiency of the stepper is poor because the operation sequence must be carried out every time the mercury-vapor lamp is changed.
It is possible to conceive a method of measuring the illuminance by providing a photodetector device at the position of the chuck so that the illuminance and illuminance distribution on the actual wafer surface can be measured. However, a distance between the reduction lens and the chuck is extremely small, and a generally available photodetector device is too bulky to fit between the reduction lens and the chuck. It is also possible to conceive a method of measuring the illuminance by providing a plurality of photodetectors at the position of the wafer, but as described before, the characteristics of the photodetectors are not perfectly identical among the independent photodetectors, and there is a problem in that it is impossible to obtain from the measured results the illuminance and illuminance distribution which are highly reliable. Furthermore, even the smallest photodetector available has such a light receiving surface with a square area of 2.5 mm to 4 mm per side that is not small compared to the exposure region which has a square area of 10 mm to 14 mm per side, and it is impossible to measure the illuminance within a small region.